Multi-key devices exist that include a plurality of keys which are activated by FSR force sensors the resistance of which varies as a function of the force applied to the key and the sensor. The device includes an algorithm permitting determination that a key has been pressed and activated when the amplitude of the applied force is greater than a predetermined force threshold.
One algorithm currently used is based on a high-pass filter having a long time constant (16 samples for a sampling period of 20 msec). The output value of this filter, which depends on the velocity and on the actuation force, is compared with thresholds to detect any change of the sensor.
Another currently used algorithm is based on a software task (long period) responsible for calculating an inactive state for the keys. On the basis of the level of the evaluated inactive state, a second software task (quick period) compares a current pressure level with the level of the recalculated inactive state.
However, it happens that the currently used algorithms do not permit correct identification of the keys pressed when a user presses on a plurality of different keys of a multi-key device in a connected manner. When a user presses on several different keys, a pressure can be applied to the force sensors of other keys on which the user has not directly pressed (particularly when the space between the keys is small), and these force sensors then provide signals to the device indicating that their keys have been pressed while in reality the user has only pressed on adjacent keys.
In addition, when a user presses heavily and/or for a long time on a key, the force sensors of the adjacent keys can receive a large pressure and provide a signal to the device indicating that their keys have been pressed.
Thus, the device does not correctly determine the keys which have really been touched and pressed by the user and, therefore, does not correctly determine the keys which the user wished to activate.